1. Field of the Invention
The present invention relates to a variable magnification optical system for projection and a zoom lens for projection that are mountable on projection-type display apparatuses or the like, and the projection-type display apparatuses. Particularly, the present invention relates to a variable magnification optical system for projection and a zoom lens for projection, and projection-type display apparatuses suitable to project images onto large screens at movie theaters or the like.
2. Description of the Related Art
Conventionally, projector apparatuses (projection-type display apparatuses) using light valves, such as a liquid crystal display device and a DMD display device, and which have relatively long back focus, were widely used.
In recent years, movie theaters or the like started using projector apparatuses that can project precise images appropriate for large screens.
Such projector apparatuses that can project high-definition images adopt a three panel method using a reflective liquid crystal display device, a DMD or the like. In such projector apparatuses, longer back focus, and excellent telecentricity similar to ordinary projector apparatuses are requested.
Generally, a value obtained by dividing a projection distance by the width of a screen is called as a throw ratio. The size of a screen and a distance from the screen to a projection booth may be different at every movie theater. Therefore, lenses corresponding to the throw ratios of respective movie theaters should be used to project images in appropriate sizes. However, when the cost of a lens is considered, it is not desirable to prepare a lens for each movie theater. Such a problem may be solvable by using a variable magnification optical system, which can be used at movie theaters with a certain range of throw ratios.
Conventionally, various kinds of zoom lens for projection that are mountable on projector apparatuses and that include two movable lens groups were known. As a lens that can project images in an excellent manner, a zoom lens composed of four lens groups, two lens groups of which are movable, is known.
A zoom lens for projection disclosed in Japanese Unexamined Patent Publication No. 10 (1988)-123419 (Patent Document 1) is composed of a positive first lens group, a negative second lens group, a positive third lens group, and a positive fourth lens group, which are arranged from the magnification side of the zoom lens in the order mentioned above. In the zoom lens, the second lens group and the third lens group are moved along the optical axis of the zoom lens when magnification is changed.
Meanwhile, a zoom lens for projection disclosed in U.S. Pat. No. 6,204,976 (Patent Document 2) is composed of a negative first lens group, a positive second lens group, a positive third lens group, and a positive fourth lens group, which are arranged from the magnification side of the zoom lens in the order mentioned above.
In the zoom lens, the second lens group and the third lens group are moved along the optical axis of the zoom lens when the magnification is changed.
As other zoom lenses for projection, zoom lenses for projection disclosed, for example, in Japanese Unexamined Patent Publication No. 2006-039034 (Patent Document 3), U.S. Pat. No. 6,741,398 (Patent Document 4), Japanese Unexamined Patent Publication No. 2007-156268 (Patent Document 5), U.S. Pat. No. 6,989,939 (Patent Document 6), Japanese Unexamined Patent Publication No. 2001-215410 (Patent Document 7), Japanese Unexamined Patent Publication No. 2001-141999 (Patent Document 8), and U.S. Pat. No. 7,911,705 (Patent Document 9) are known. The zoom lens for projection disclosed in Patent Document 3 is composed of a negative first lens group, a negative second lens group, a positive third lens group, a positive fourth lens group, and a positive fifth lens group, which are arranged from the magnification of the zoom lens in the order mentioned above. In the zoom lens, the second lens group, the third lens group and the fourth lens group are moved along the optical axis of the zoom lens when the magnification is changed.
Patent Document 4 discloses zoom lenses for projection including at least one diffraction optical element. The zoom lens for projection in Patent Document 4 is composed of a negative first lens group, a negative second lens group, a positive third lens group, a negative fourth lens group, and a positive fifth lens group, which are arranged from the magnification side of the zoom lens in the order mentioned above. Further, Patent Document 4 discloses a zoom lens for projection composed of a negative first lens group, a negative second lens group, a positive third lens group, a negative fourth lens group, a positive fifth lens group and a positive sixth lens group, which are arranged from the magnification side of the zoom lens in the order mentioned above.
Further, Patent Document 5 discloses a zoom lens for projection composed of a negative first lens group, a negative second lens group, a positive third lens group, a positive fourth lens group, and a positive fifth lens group, which are arranged from the magnification side of the zoom lens in the order mentioned above. Further, Patent Document 5 discloses a zoom lens for projection composed of a negative first lens group, a negative second lens group, a positive third lens group, a positive fourth lens group, a negative fifth lens group and a positive sixth lens group, which are arranged from the magnification side of the zoom lens in the order mentioned above.
In the zoom lens for projection disclosed in Patent Document 1, the first lens group on the magnification side has positive refractive power. Therefore, a high zoom ratio is achievable in a telephoto type lens system. However, when the projection angle of the zoom lens for projection is increased without lowering the zoom ratio of the zoom lens, it is impossible to obtain sufficient back focus, or the outer diameter of at least one lens in the first lens group on the magnification side of the zoom lens becomes too large, and it becomes difficult to provide a compact lens system.
In the zoom lens for projection disclosed in Patent Document 2, the power of the lens groups arranged from the magnification side of the zoom lens is negative, positive, positive, and positive.
Further, the power of the lens groups that move when the magnification of the zoom lens is changed is positive and positive. Therefore, when the magnification ratio becomes large, the fluctuation of aberration becomes too large. Hence, it is difficult to substantially increase the magnification ratio.
The zoom lens for projection disclosed in Patent Document 3 has short back focus, and a small magnification ratio. Further, the zoom lens for projection disclosed in Patent Document 4 has a small magnification ratio. The zoom lens for projection disclosed in Patent Document 5 has short back focus. Further, in the zoom lenses for projection disclosed in Patent Documents 3 through 5, the fluctuation of numerical aperture when magnification of the variable magnification optical system is changed is not considered. Therefore, when the magnification of the variable magnification optical system is changed, the numerical aperture fluctuates, and such zoom lenses for projection are not appropriate for movie theaters.